Update documentation related to CentOS corporate visual identity.
    
    - Previously, the render.conf was pointing to messages.po as common
      source for localization messages both for corporate.asciidoc and
      corporate.svgz files. This update changes render.conf file to
      use the corporate.asciidoc.po and corporate.svgz.po files instead.
    
    - Fix path related to corporate.png image inside corporate.asciidoc.